Object detecting and locating apparatus



Dec. 15, 1964 s. M. vosus ETAL 3,161,351

OBJECT DETECTING AND LOCATING APPARATUS Filed Ila.

l1 Sheets-Sheet 6 Seq axim/E1 m OS mu O mv M Y. R O G E R G VICTOR F. WILLIS ATTORNEY Dec. l5, 1964 G. M. VOGLIS ETAL OBJECT DETECTING AND LOCATING APPARATUS 1 I ll Sheets-SI met 8 Filed May 31, 1957 M m ms US 6 l W U i J .W 2206 M all z ow Y r R R mi W VI. m R I mm; 3152 Gv m; Y B lmwb E m IHHM. mmE Qz m m; m M. Q Q 2 Q m a In do: fiawvwoo do: #3 woo I 82 QSWQVKS do: um v waswoo do: m 6 856m 3m .6 womoom 3m 6 wumnom 5 6 M858 Zm 60h N9. 60 5E Ez 5E5? $13152 5153 mwi flq u LE LE LE1 LE 62 E8355 mmozomzfit 58855 EoBmzsE $85245 52+ zt+ 3 f I moh MON [3 62 on ATTORNEY Dec. 15, 1964 G. M. VOGLIS ETAL OBJECT DETECTING AND LOCATING APPARATUS ll Sheets-Sheet 10 Filed May 31. 195'! m hHl muhIm KMELIIm mmqIm INVENTORS GREGORY M. VOGLI S VICTOR F. WI LLIS ATTORNEY United States Patent I 3,161,851 OBJECT DETECTING AND LOCATING APPARATUS Gregory M. Voglis, Teddington, and Hector F. Willis,

London, England, assignors to the Commissioners for Executing the Office of Lord Admiral of the United Kingdom of. Great Britain and Ireland Filed May .31, 1957, Ser. No. 662,942

6 Claims. (Cl. 340--6) This invention concerns apparatus for the rapid detection and location of objects by directional reception of energy emitted by or reflected from the objects, and,

although the invention is not necessarily applicable only to apparatus responsive to acoustic energy, the invention was primarily developed for the detection and location of submerged objects by the use of acoustic energy, after the manner .of the now well-known ASDIC apparatus, so that, for convenience, the following description will be mainly concerned with the acoustic aspects of the invention. For further description of the abovesaid ASDIC apparatus reference is bad to the section entitled Sound Underwater in Scientists Against Time by Baxter which was published by Little, Brown and Company, Boston, 1946.

In the acoustic field the reception of sound and its conversion into electrical energy is effected by a transducer, and the ability of any transducer to discriminate between sounds from different directions is represented by the beamwidth which is defined as proportional to the ratio A/D where 7\ is the wavelength of the sound in the medium (eg. liquid) in which the transducer is immersed, and D is the length of the transducer face.

Ordinary forms of transducer receive effectively only when the transducer is facing directly towards a sound reflecting or sound emitting object, within an arc corresponding to the beamwidth, so that the transducer has to be turned bodily to search over a larger'arc. This way of scanning cannot be used for fast searching because of physical difiiculties and the limitation imposed by the time required for sound to travel from source to receiver, i.e., to the maximum range and back in the case of echo reception.

The need for moving the transducer bodily can be avoided by using a multiple element strip transducer and suitably modifying the phase relationships of the separate outputs from the individual elements before combining them, so as to scan by altering the direction of maximum response of the transducer and obtain the electrical equivalent of turning the transducer bodily without actually moving it. The phase changes required to produce this effect have in previous systems been effected discontinuously so that the direction of maximum sensitivity of reception is deflected in steps.

The present invention is an improvement on the aforesaid eleotrical scanning and enables continuous and rapid deflection of the direction of beamed reception to be obtained in a simple manner, so that it is inherently capable of very high speed operation. It can be applied to apparatus using a highly directive sound receiver in conjunction with a suitable local transmitter from which sound is emitted over a wide angle which is scanned by the receiver; it may also be applied to the scanning of sounds emanating from distant sources over a wide sector.

The method used has been designated modulation scanning and is based on the fact that a frequency change imparted to any signal is'equivalent to a phase modulation of that signal which is linear with respect to time. More explicitly, suppose that the original signal of frequency f is subjected to a frequency change, the result being a signal of an amplitude proportional to that r 3,161,851 Patented Dec. 15, 1964 ice of the original signal and of frequency f +Af where A is the frequency increment. It now this resultant is compared with its original form, it can be interpreted as a signal at frequency f the phase of which increases linearly with time, the maximum phase deviation being 21r changing at the rate of '21rAf rad/sec. As the modulation scanning speed is determined by this rate of phase change, it follows that it is entirely dependent on the frequency change M.

The application of this principle to transducers follows simply. If the transducer consists of N elements and by some means the frequency of every signal obtained from the elements is changed so that the resultant frequencies form the sequences Where f f if, i is the acoustic frequency, f a frequency of any value including 0, and f the scanning frequency, then the added resultants represent mathematically a beam' of 'beamwidth equal to that of the entire transducer, scanned periodically but continuously over a sector equal to N times the beamwidth at the rate of f scans per see, the direction or bearing of maximum sensitivity within the scanning sector being at every instant uniquely determined by the phase of the scanning cycle i.e., as harmonic functions of time.

In more general terms, any method of frequency change or modulation, which reduces a set of voltages of the form to ,asingle function of the type where a and b are constants and N is either 2n+1 or 2n, can be regarded as a practical realization of the basic principle of modulation scanning.

The invention accordingly comprises apparatus for directional reception of sound or supersonic or subsonic energy propagated in a fluid and for periodic uniform and continuous deflection of the direction of maximum sensitivity of reception at a selected scanning frequency, comprising a transducer immersed in the fluid and subdivided into elements, means of coherently modulating the outputs of each element and of processing their resultants so as to introduced between the outputs of any element and its adjacent element to one particular side, effective and coab cos [21r(foi ffil herent phase shifts linearly varying with time but always the same for all adjacent pairs of elements at every instant of the modulation'cycle, means of adding the processed modulation products to produce a single carrier modulated electrical signal of carrier frequency smaller or larger than or equal to the acoustic frequency and of carier amplitude proportional to the magnitude of the sound appropriate to the deflection of the beam at any instant, and means of displaying on a C.R.O. screen the signal thus derived so as to determine, from the phase of the scanning cycle at the instant of maximum carrier amplitude response, the direction of bearing of a transmitting or reflecting source of the received sound relative to the normal of the transducer face.

Various ways of carrying out the invention are illustrated in the accompanying diagrams. They fall into two main categories:

Perhaps a better understanding of the invention can be obtained by comparison of the signals derived from an analogous mechanically scanned system. Imagine for example a horizontal array consisting of an odd number of transducers lying in a vertical reference plane and rotatable about a vertical axis in the reference plane through the center transducer. Now imagine that the array is ro tating, and has just assumed a position coincident with the reference plane at the instant that a plane wavefront is passing through the reference plane. In this position the transducers, with the exception of the center transducer, are moving parallel to the direction of propagation of the Wavefront; those to one side of center moving with the wavefront and those on the opposite side moving in the opposite sense. The velocity of each transducer according to simple mechanics relative to that of the reference plane is proportional to its distance from the center. The pair of transducers nearest the one at the center, thus, effect a Doppler frequency shift A in the passing wavefront which according to wave mechanics is nearly proportional to the velocity of that transducer for small changes in frequency. The next nearest transducer being twice as far from the center effect a frequency shift 2A the third 3A1, etc. The center being stationary'produces no Doppler. Transducers placed between those in the above array to form a symmetrical even numbered array would obviously effect intermediate values of A3 such as AA Af, etc. The present invention produces an apparent rotation by heterodyning the signals from an array with a spectrum of locally generated synchronous signals with frequencies corresponding to the values of A as set forth above.

Even though the signals at each of the transducers are different in a mechanically rotated array there is a certain phase coherence between signals. For example, if the entire array were stationary and instantaneously brought to scan rotation velocity, a cycle of each A) would start at the same instant. This phase coherence must also be present in a heterodyne processing or steering system.

Thus coherence does not require rephasing of the locally generated synchronous signals, but means rather that care should be taken to avoid unnecessary shifts in the relative phase of the signals processed.

A more thorough analysis indicates the simulation of one or more, depending on the scan frequency, rotating arrays spaced at angles to one another, but only the one in closest coincidence with the reference plane of the transducers is effective. This is due to the limits placed on the input signal available for processing by the directivity of the fixed transducer structure.

The active period of each simulated transducer is one cycle of the lower A which may therefore be used as a time base.

The invention is best understood with reference to the accompanying drawings, wherein:

FIGS. 1-5 show different circuit embodiments of one class of scanning system for producing an output centered on the acoustic frequency according to the invention;

FIG. 5A is a phase vs. frequency graph of the output signal components of the circuit in FIG. 5;

FIGS. 6-9 show different embodiments of a second class of scanning systems for producing an output at a frequency other than the acoustic frequency according to the invention;

FIG. 10 shows an embodiment of a third class of scan ning systems according to the invention; and

Single Stage Modulation Scanning FIG. 11 shows a diagram of the beam pattern produced by the scanning system of FIG. 10.

Referring to FIG. 1 a simple form of the invention is shown. As will be the case with all of the species disclosed herein the array consists of a plurality of N transducers one of which may be centered and will be called the Zeroth transducer 101. On each side of the center are n transducers, the nth transducers 104 and 105 being at the ends of the array and remaining rth transducers such as 102 and 103 arranged in ascending order from the center to each end. The term r is merely an integral number denoting the order of the transducer and should not be confused with 7 which shall be discussed presently.

With the exception of the center transducer the acoustic signals received by each of the transducers becomes an electrical signal and is applied to the input of a balanced modulator, as for example 110. Each of these same input signals is also passed through a quater-wavelength phase shifter such as 107 and applied to the input of a second balanced modulator 108. A preamplifier, such as 106, will generally be required at each transducer to provide sufficient drive for the modulator.

The phase shifted signals to one side of center are modulated by a local signal source 109 with a waveform sin Fro t. The same signals on the opposite side are modulated by a similar signal inverted in phase. The factor F is a constant which is difli'erent for each pair of transducers symmetrically arranged about the center of rotation of the array as indicated by the subscripts in the drawings and may be simply defined in an equally spaced transducer array as the ratio of the distance from center divided by the transducer spacing. The term m is the de sired angular scanning frequency of the array. The r'e maining non-phase-shifted signals are modulated with a source 111 of cos Fw t. All of the signals are then ap plied to a common adder 113.

Although the resultant signal from the adder would appear to be quite complex an examination of the inputs reveal that this is not so. The use of balanced modulators eliminates the original acoustic frequency as an output. The phase shift effected, for example by element 107, together with the phase difference of the modulating signals eliminates one sideband. The fact that one set of modulating signals is inverted results in the elimination of the low frequency sidebands on one side and the high frequency sidebands on the other. Thus the remaining signals are just those discussed above in the mechanically rotated array.

The resultant adder signal may then be passed through a filter 114 with a minimum band pass of (N 1)f +df where N is the number of transducers, f the scanning frequency and df the bandwidth of the transducer; the center frequency of the filter being the input acoustic frequency. The signal may also be passed through an amplifier 115, if required, to drive a display means, Usually the display means will be a cathode ray tube 11 as in the ASDIC apparatus mentioned above, and the signal operates the intensity grid, although it could equally well be applied to another control element. A source 118 of scan frequency signal is applied to one set of deflection plates 117 to provide a time or angular base for the information displayed.

FIG. 2 shows a structure which achieves the same results as that described above but with different elements. The transducers 201-205 are the same as before, as is the preamplifier 206. There is however only a single set of balanced modulators 207 one of which is driven by each of the transducers. The modulation signal for each is taken from a source, such as source 203, which in every case has the waveform cos Fe t. The modulation; product signals for one side of the array are combined, in a first adder 209, and those on the other side in a, second adder 210. The output of adder 209 passes, through a high pass filter 211 which has a. cutoff. above.

the acoustic input frequency f0, i.e. an attenuation of 34-40 db at and the output of adder 210 through a low pass filter 212 with a similar cutoff at the frequency The outputs of filters 211 and 212 are then combined by an adder 213. From this point the signal passes through a filter 214, an amplifier 215, and display elements 216-218 all of which correspond to elements 114- 118 in FIG. -1.

The operation of the FIG. 2 device differs only in the means of disposing of the unwanted sidebands from the modulators. The filters 211 and 212 remove the lower and upper sidebands from the signals on each respective side. This requires an extra adding step in elements 209 and 210 because the desired and undesired sidebands on opposite sides are identical or overlap making filtering impossible, if they were all previously combined.

FIG. 3 shows a further embodiment for achieving the results obtained in the structures described above. Again the transducers 301-305 are arranged as before and each may have a preamplifier 306. The output of each preamplifier contains a divider circuit 307 or 308 which produces two equal signals. To one side of center a half output of each divider is passed through a separate quarter-wave phase shifter 309 and thence drives a balanced modulator 310. The signal is there modulated by a source 312 of sin Fw t. Similarly a half output from each transducer on the opposite side is fed without phase shift to a separate balanced modulator, for example element 311, and modulated by a source 313 of cos Fw t. The remaining half-output from each divider is combined in antiphase relationship with the previously mentioned half-output of thecomplementary transducer equally spaced from the center of the array on the opposite side. As shown a transformer type divider is well suited for this purpose. The modulator outputs are then combined in an adder 314 and may be filtered by element 315, boosted in amplifier 316 and displayed by elements 318-319 which correspond to exactly the same elements found in FIG. 1.

Although it .is not immediately apparent the action of this circuit is quite similar to that in the FIG. 1 device. If one considers that a signal is received only by a transducer on one side such as element 302 there are two modulator branches for this signal through elements 310 and 311. Similarly, if transducer 303 receives a signal it passes through the same two branches. The source of sin Fw t is inverted 'onone side of the array in FIG. 1, but this is compensated in the FIG. 3 device by the antiphase signals supplied by the dividers 307 and 308.

FIG. 4 is closely related to the structure in FIG. 3. The transducers 401-405, the preamplifiers such as 406, the dividers 407 and 408, and the modulators 409 and 410 are interconnected in the same manner. Missing are individual 1r/ 2 phase shifters which might be expected to precede modulators such as 410. The signals from all the modulators to one side including modulator 409 are fed to an adder 414, as is the unaltered signal from the center transducer, if present. The signals from the remaining modulators are fed to a second adder 413. The outputs of the latter elements feed a network 415 which shifts all of the signals from adder 413 a quarter wave in phase relative to the signals from adder 414 and combines the result, obviating the need of the separate phaseshifters 309 of FIG. 3. Elements 416-420 are the same as the related elements described in previous figures.

In FIG. 5 the elements 501-514 are identical instructure and function to those described in FIG. 4, with the exception of the source 511. In this embodiment the signals on both sides of the array are modulated by a source of cos Fe t. The output of adder 514 is applied to a phase equalizer network 516 which imparts a constant phase shift vs. frequency characteristic oz to the signals emerging from one side of the array. The curve 41 of this function is shown at the bottom of FIG. 5A. The output of adder 513 is applied to a second phase equalizer 515 with a cross-over characteristic centered at the acoustic input frequency, as shown by the curve (15 111 FIG. 5A. Due to the cross-over the high frequency-sidebands from the adder to the left are shifted in phase 180. The transducers to the left feed the adders on both sides of the array modulation products that are in phase so that both sidebands would ordinarily reinforce each other. The equalizer cross-over reverses this condition for the upper sidebands which are thus eliminated. The transducers to the right feed the adders similar signals in antiphase relationship and would normally cancel one another. Again this condition is reversed by the equalizers for the upper sidebands which are thus retained. Adder 517 combines the remaining sidebands and the elements 518- 521 may be used to display the results as previously.

FIG. 6 shows a scanning system which can be used over a wide range of acoustic input frequencies. Like the system previously described it contains a similar group of transducers 601-605 each of which may have a preamplifier 606. The signals feed a heterodyne processing circuit 600 which can be chosen from any of the five preceding species such as in FIG. 1, 200 in FIG. 2, etc. The acoustic signal from each transducer in this arrangement, however, passes through a different balanced tuning modulator 607 and a band pass filter 609 before it is applied to the processing circuit 600. In element 607 the input signal is modulated by a source 608 of cos wt. The value of w depends on the input acoustic frequency and the intermediate input frequency of the heterodyne processing circuit. Thevalue of w is chosen so that its sum or difference with the acoustic input frequency equals the intermediate frequency. Bandpass filter 609 is tuned to the intermediate frequency and has the same bandwidth as the transducers. With this arrangement only to needs to be adjusted for a change in acoustic frequency and retuning of the bandpass filters is unnecessary. Elements 610-614 may be the same as corresponding structure in preceding embodiments.

The embodiment of FIG. 7 utilizes a similar array of transducers 701-705 each with its own preamplifier 706 feeding a balanced modulator 707. The center transducer is modulated by asource 709 of cos wt. The transducers on one side of the array are each modulated from a source of cos (w+i "w )t, while those on the other side by a source of cos (w?w )t.

This provides the proper components necessary to an apparently tilting array each of the components having been translated above and below the acoustic frequency by the factor w. These components are combined in an adder 711 and passed through filter 712 having a bandpass equal to filter 114 in FIG. 1. The bandpass is centered on either the sum or difference of the acoustic frequency and w. Again the only adjustment required for changes in the acoustic frequency is a change in w. Elements 713-716 correspond to similar elements in preceding embodiments.

FIG. 8 employs the same array 801-805 of transducers each with a preamplifier 806 as described in previous embodiments. The output of each preamplifier is passed through a divider, as elements 807 and 808, and fed to symmetrically located modulators 810 and 811 as taught in FIG. 3, for example. The modulators on one side of the array are connected to a source of sin wt sin Fw t, while those on the opposite side are fed from a source of cos wt cos Fw t. The center transducer, if present, is

7 provided with a modulator 809 connected to a source of cos wt.

Comparison with the structure of FIG. 8 with that in FIG. 3 shows that the modulation products are similar except that there are two sets in FIG. 8 separated in frequency from those in FIG. 3 by the factor to. The components are combined in adder 815 and passed through a filter 816 with a bandpass similar to filter 114 in FIG. 1. The center frequency of this filter is tuned to either the sum or difference of the acoustic input frequency and w so that one set of resultant components is obtained. Elements 817-820 may be used to display the result as in previous embodiments.

FIG. 9 may be compared in a similar manner to FIG. 4. The circuits are identical in elements 910-911 with the exception of modulator MW provided for the center transducer. That modulator is connected toa source 912 of cos wt. The modulators to the left of center are connected to a source of cos wt sin Fw t while those to the right are connected to a source of cos wt cos Fw t. The cos wt function again provides sets of signal components like those in FIG. 4 but separated above and below them in frequency by the factor w. The signals are added one each side as before, one side is shifted a quarter wave in phase over the frequency band of filter 918 and added in the circuit 917, thereby canceling undesired components in the spectrum of the output signal. Bandpass filter 918 is tuned to have a center frequency at the sum or difference between the acoustic frequency and w and has the same pass band as filter 114 in FIG. 1. Display elements 919922 are used as previously.

The various components for the systems heretofore described are all well known elements, Which have been developed extensively in the prior art and are discussed in standard textbooks on audio and radio engineering. The only problem which might present itself in circuits of this type would be that of providing bandwith for separation and filtering the various signals. With the frequency translation systems using sin wt signals provided the problem of bandwidth is no longer a limitation since the bandwidth may always be made a small percentage of the signals in various parts of the system.

FIG. 10 shows a scanning system which employs a combination of the structures described in the preceding figures. This system employs an array which is divided into a plurality of subgroups of 11 transducers, as for example the transducers ltltll, 1W2 and 1003. The signals from each group are fed to a separate heterodyne processing circuit 1064 chosen from any of the forms previously described. The frequency translation structure described in FIGS. 69 may also be employed with the heterodyne processing structure, but is omitted for the purpose of simplifying the discussion which follows. The output of each of the heterodyne processing units passes through a bandpass filter corresponding to those employed in the systems previously described.

The outputs of these filters form the inputs for a second stage or heterodyne processing circuit 1008. In designing the second stage processing circuit each filter output from the first stage including elements 1004 and 1007 is treated as a fictitious transducer input and the scanning frequency is assumed to be n f Filter 1%? is designed on the bandwidth and scanning frequencies thus defined in the same manner as filter 114 from FIG. 1. It is obvious to those skilled in the art that, since the bandwidth of filter 16437 is greater than the transducer 1001 and the scanning frequency of the second stage is increased, the frequency translation structure of FIGS. 6-9 will generally be desirable in the second stage. The output signals from filter 1009 may be used with display elements mid-M13 as described in previous embodiments.

The principle of two stage modulation scanning can be readily extended to any number of stages, for the g outputs of the first modulation stage may be divided again groups of n elements each. Each group is now scanned separately but coherently at the scanning frequency n in a similar manner as in the first stage of a two scanning process. There will be g independent second stage scanned outputs which can be divided again into groups and scanned at the frequency n j This process can be extended to any number of stages without affecting the final result. For it can be proved mathematically that, provided at any stage, consisting of one or more groups, the scanning is carried out at a frequency equal to that of the proceeding stage times the number of elements contained in each of its groups, the final output obtained from a multistage scanning process is identical to that derived from a single stage scanning method, the parameters N and remaining the same.

Multistage modulation scanning shows many advantages over single stage methods, the most important being a substantial reduction in the number of modulating voltages required to operate the scanning gear. This saving is particularly pronounced when N is a large number i.e. a narrow beam is to be scanned over a large sector. In all such cases multistage modulation will result in a worthwhile reduction in the size of the scanning gear as compared with that obtained by application of single stage modulation scanning methods.

The number of stages to be employed in a multistage scanning process, will largely depend on the magnitude of N but in any case no requirements for more than 4 stages of modulation are likely to occur in practice.

The multistage structure described above affords the possibility of scanning an acoustic beam in two alternative modes of operation.

Consider again an N element transducer scanned with the frequency f,. If single stage modulation is employed the sector scanned is equal to approximately N times the receiver beamwidth and therefore fixed. On the other hand, if the beam is scanned by means of multistage modulation, there are two alternative modes of operation, namely the beam can be scanned either (1]) Over the complete scanning sector at the frequency 5 or (2) over l/n th of it at the rate of 11 .1 scans per second,

(where n represents the number of elements in the first stage subtransducer).

The changeover is effected by arranging the modulating voltages to the first stage modulator so that each first stage subtransducer output is proportional to the vectorial sum of all n voltages induced in the elements of one subtransducer. In practice this can be accomplished by substituting for the AC. modulating voltages to the modulators of the type cos Fwst of FIGS. 1, 3, 4, 8, 9 a direct current of suitable magnitude and short circuiting the modulation terminals of all remaining modulators, whereas in the case of the method represented by FIG. 7, the modulating voltages to all modulators are made equal to that of the center element i.e. cos wt. Scanning methods of FfGS. 2 and 5 are not suitable for this type of scanning.

The two modes of operation are shown in FIG. 11, the alternative scanning sectors being scanned at the frequencies f and n j respectively.

In normal operation sector oi will be scanned but if an enhancement in detail of the central part of the sector is required, mode 2 and can be employed resulting in an scanning rate.

What we claim is:

1. Apparatus for controlling the directional reception of compressional Waves in a fluid medium by an array of transducers, comprising:

a plurality of modulating means each coupled to a different one of said transducers for modulating output signals therefrom at a frequency proportional to the spacing of the center of said one transducer from a preselected reference point in said array, thus producing upper and lower sideband signals for each transducer,

said modulating means comprising two modulators connected in parallel to each transducer; a quarterwave length phase shifter interposed in the input path of one of said modulators; and each modulator being connected to a separate source of modulating signal, said sources being synchronized in phase quadrature;

and a first adder means coupled to said modulating means for directly combining all of said sideband signals. from at least half of the transducers having their centers spaced from said preselected reference point.

2. Apparatus for controlling the directional reception of compressional waves in a fluid medium by an array of transducers, comprising:

a plurality of modulating means each coupled to a different one of said transducers for modulating output signals therefrom at a frequency proportional to the spacing of the center of said one transducer from a preselected reference point in said array, thus producing upper and lower sideband signals for each transducer.

said modulating means consisting of a single separate modulator coupled to each of said transducers;

a first adder means coupled to said modulating means for directly combining all of said sideband signals from at least half of the transducers having their centers spaced from said preselected reference point;

a second adder means for combining the signals from the modulators connected to the remaining of said transducers having their centers spaced from said preselected reference point;

a'low pass filter means coupled to said first adder for excluding all signals higher in frequency than those of said lower sideband;

a high pass filter means coupled to said second adder for excluding signals lower in frequency than those in said upper sideband;

and a third adder for combining any remaining signals from all of said transducers.

3. Apparatus for controlling the directional reception of compressional waves in a fluid medium by an array of transducers, said array including a plurality of pairs of transducers, the transducers of each pair being equally spaced from a preselected reference point in said array, comprising, in combination:

a plurality of modulating means each coupled to a different one of said transducers for modulating output signals therefrom at a frequency proportional to the spacing of the center of said one transducer from said reference point, thus producing upper and lower their centers spaced from said preselected reference point.

4. The apparatus according to claim 3 wherein; a quarterwave phase shifter is interposed in the input of one modulator in each of said pairs of modulators, said sources are synchronized in phase quadrature and said first adder means combines the outputs of all of said modulators directly.

5. The apparatus according to claim 3 wherein the output of one modulator in each of said pairs of modulators is coupled directly to said first adder; a second adder is coupled to the output of each remaining modulator; the modulation source coupled to said one modulator being synchronized in phase quadrature to the source coupled to said remaining modulator; a broadband phase shifter and adder means coupled to the outputs of said first and second adder means for shifting the phase of signals from said first adder output a quarter-wavelength relative to corresponding signals from said second adder output and combining said signals.

6. The apparatus according to claim 3 wherein the outputs of one modulator in each of said pairs of modulators is coupled directly to a first adder; a second adder is coupled to the output of each remaining modulator; each modulator being connected to a source of in-ph-ase synchronized modulating signal; a third adder; phase equalizer means coupling said first and second adders to said third adder for shifting the phase of said upper sideband signals from said first adder a half wave-length relative to said upper sideband signals from said second adder.

References Cited by the Examiner UNITED STATES PATENTS 2,166,991 7/39 Guanella' 343--115 2,225,928 12/40 Ring 343100.6 2,247,666 7/41 Potter 343100.6 2,403,728 7/46 Loughren 343-1006 2,426,460 8/47 Lewis 343- 2,429,726 10/47 Lewis 343-100 2,430,296 11/47 Lewis 343- 2,437,281 3/48 Tawney 343-400 2,464,276 3/49 Varian 343-1006 3,012,244 12/61 Langenwalter et a1. 343-415 CHESTER L. JUSTUS, Primary Examiner. NORMAN H. EVANS, Examiner. 

1. APPARATUS FOR CONTROLLING THE DIRECTIONAL RECEPTION OF COMPRESSIONAL WAVES IN A FLUID MEDIUM BY AN ARRAY OF TRANSDUCERS, COMPRISING: A PLURALITY OF MODULATING MEANS EACH COUPLED TO A DIFFERENT ONE OF SAID TRANSDUCERS FOR MODULATING OUTPUT SIGNALS THEREFROM AT A FREQUENCY PROPORTIONAL TO THE SPACING OF THE CENTER OF SAID ONE TRANSDUCER FROM A PRESELECTED REFERENCE POINT IN SAID ARRAY, THUS PRODUCING UPPER AND LOWER SIDEBAND SIGNALS FOR EACH TRANSDUCER, SAID MODULATING MEANS COMPRISING TWO MODULATORS CONNECTED IN PARALLEL TO EACH TRANSDUCER; A QUARTERWAVE LENGTH PHASE SHIFTER INTERPOSED IN THE INPUT PATH OF ONE OF SAID MODULATORS; AND EACH MODULATOR BEING CONNECTED TO A SEPARATE SOURCE OF MODULATING 